1. Field of the Invention:
The invention relates generally to improvements in pulse width modulated voltage regulator-converter systems having balanced "current mode" transformer drive means, and, in particular, to a voltage regulator-converter/power converter system having increased output power capability.
2. Description of the Prior Art:
Various types of switching regulator systems have been developed in the past, and, in many applications, they are superior to conventional series regulator systems. It is well known that conventional series regulator systems operate as a variable resistor which drops an unregulated input voltage or bulk voltage down to a fixed output voltage. The element utilized, generally, to drop the voltage down is a power transistor. Accordingly, inter alia, the aforesaid power transistor must be capable of dissipating the power created by the voltage difference between the input or bulk voltage and the fixed output voltage at the load current. It is apparent that the power generated can become excessive, particularly, when the input or bulk voltage is not well regulated and the difference between the input and output voltages is large. Consequently, switching regulator systems have been found to be superior in applications where the output voltage of the system is considerably lower than the input or bulk voltage. It follows therefore, in aerospace systems, for example, that a missile with a 28 volt battery as its only power source containing a large number of integrated circuits which require a 5 volt supply can benefit from the utilization of a switching voltage regulator. In these applications, switching regulators, due mainly to their higher frequency of operation, are the most economical and reliable solution to power supply needs since power is conserved and the weight of transformers and filter inductors are reduced.
Notwithstanding the improvements aforementioned, there is and has been a need to further conserve power consistent with good regulation and high output power capability. Accordingly, U.S. Pat. No. 3,670,234, filed June 30, 1970, by James M. Joyce, entitled, "Pulse Width Modulated Voltage Regulator," and assigned to the same assignee as this application, discloses a voltage regulator-converter circuit that has improved efficiency and utilizes light weight components. Joyce accomplishes his objects by utilizing, inter alia, an "interstage current mode transformer" thereby, improving the performance of a standard push-pull power switching regulator. In Joyce's system, a pulse width modulated voltage gates a control means which holds a pair of driver transistors in a conductive state during the dwell period of the modulated voltage. This causes a short across the primary side of the "interstage current mode transformer," thereby, switching the power amplifier stage off completely allowing energy stored in the primary side of the "interstage current mode transformer" to be dissipated eliminating the possibility of the discharge of stored energy in the secondary of the aforementioned transformer. If the stored energy is discharged in the secondary winding, distortion of the waveforms would occur causing partial turning on of the power switching stage during the dwell time causing a decrease in efficiency. In addition, the turns ratio of the positive feedback and secondary windings connected to each power transistor is established to be equal to the desired current gain thereof providing thereby a fixed current gain for the switching transistors when they are in conduction. An advantage of this connection is reduced storage time in the power transistors, and accordingly, an additional increase in the efficiency. Also this connection, to a substantial degree, decreases crossover distortion and eliminates critical matching of the power transistors since the current gain thereof is dependent more on controllable external circuit parameters than transistor parameters.
Hence, the "current mode transformer" connection of Joyce solves many problems inherent in prior art switching voltage regulators. Consequently, this aspect of the Joyce invention is being utilized in the instant invention and will be more fully discussed in conjunction with the "Statement of the Operation" hereinafter. Nevertheless, there are problems in the prior art which have not been solved by the voltage regulator-converter of Joyce. These problems become more manifest when switching regulators are to be utilized in ground electronic systems on a worldwide basis. For example, the vast majority of ground electronic systems are designed and built to plug into a wall outlet. Accordingly, the AC voltage at the wall outlet varies, typically, depending on the country of location, from 100 volts RMS to 235 volts RMS. In the past, it was common to utilize 50 or 60 Hz step down transformers with secondary series regulators, in some cases, or secondary switching regulators, with proper filtering, to lower the bulk input voltage to a value that could be utilized with a switching regulator like Joyce's configuration, for example. However, even in ground electronic systems, a 50 or 60 Hz transformers present size, weight and cost factor penalties that are prohibitive in a business sense. In addition, the advantage of the Joyce system, in terms of overall efficiency, is not fully utilized by using the step down transformer means aformentioned. Of course, Joyce's system will operate with bulk voltages up to approximately 150 volts which corresponds to a line voltage of about 120 volts RMS. Thus, there is a need in the prior art for a switched regulator-converter that will not only operate efficiently at line voltages up to 120 volts RMS but will also operate, with the same efficiency, with line voltages up to 235 volts RMS corresponding to a bulk voltage of up to 400 volts.
One contribution to the art, by the instant invention, can best be understood by briefly pointing out a limitation in the Joyce invention. Joyce utilizes a push-pull circuit configuration, i.e., power switching transistors connected in push-pull feeding an output transformer in push-pull. At the outset, it should be mentioned that the configuration, per se, is not a limitation, but the limitation is in the stress capability of "state of the art" power switching transistors. For example, it is believed that, when power switching transistors are being considered for use in switching regulators of the class herein discussed, V.sub.CEO (the collector to emitter voltage with the base opened) is the critical circuit parameter, notwithstanding current gain, switching speeds, etc., for reliable design. A V.sub.CEO rating in the range of 300-400 volts is the "state of the art" at the present time. Thus, assuming that a "state of the art" power switching transistor is used in Joyce, the upper limit on how high the bulk voltage can be is about 150 volts. This is true since the polarities on the output transformer are such as to increase the voltage across any one of the power switching transistors, when it is off, to two times the bulk voltage. In addition, other factors such as leakage inductance, voltage transients, etc., will increase the voltage from collector to emitter of the off transistor, even more, so that a conservative approximation would put the voltage at about 400 volts, the "state of the art" limit. Consequently, using this same rationale, a 400 volt bulk or input voltage would produce a stress voltage of about 1200, i.e., three times the maximum allowed by the "state of the art" power switching transistor at the present time. Accordingly, there is a need for a power switching regulator that has the advantages of the Joyce invention and yet be able to operate at an input or bulk voltage approximating the V.sub.CEO of "state of the art" power switching transistors.
Another prior art system disclosed in IBM TDB, Volume 17, No. 6, Nov. 1974, pages 1670-1671 to Duspiva, et al., appears to solve the problem of operating at higher input or bulk voltages by providing a switching regulator configuration having the collector of the power switching transistor tied directly to the bulk voltage in a single-ended configuration. The "current mode transformer" of Joyce is utilized as a coupling means between a pulse width modulated control means and the aforementioned single-ended switching transistor. Nevertheless, in order to ensure that the "current mode transformer" is balanced magnetically, auxiliary circuit means is provided to establish equal voltage-second products of the primary referenced from the center tap to the primary ends. Thus, while this current configuration accomplishes balanced operation of the "current mode transformer", and the advantages attributed thereto, and is capable of operating at bulk voltages approaching the "state of the art" of power switching transistors, it is accomplished at the expense of output voltage capability. This is true because in order to reset the transformer magnetically, and avoid saturation thereof, a minimum off time of one-fourth the period of the switching voltages is necessary to ensure that the transformer has been reset magnetically. Hence, the filtered output voltage, utilizing a conservative analysis of the aforementioned prior art circuit, would yield only 75 percent of the bulk voltage at the averaging filter output. Thus, there is a need in the prior art for a switching regulator that operates such as to always drive the "current mode transformer" in equal and opposite directions in a half cycle at bulk voltages approaching the "state of the art" V.sub.CEO of power switching transistors without sacrificing output voltage capability. Accordingly, in the instant invention, the only limitations on-off time and on-time of the switching power transistors are the parameters of the switching device and the parasitic parameters of the other circuit components. For example, the present invention is capable of providing an output voltage after filtering, that is 97.6 percent of the input or bulk voltage.
The prior art, as indicated hereinabove, includes many advances in pulse width modulated voltage regulator-converter systems, including improvements in output power capability, voltage efficiency, and power efficiency. However, insofar as can be determined, no prior art switching voltage regulator-converter system incorporates all of the features and advantages of the instant invention.